1. Field of the Invention
The present invention relates to a process for the preparation of coated paper and cardboard in which a coating material composed of particles of an aqueous dispersion of plastic and inorganic pigment particles is applied to and dried on the raw paper or cardboard for coating.
2. Description of the Prior Art
The requirements for the increasing high quality demanded of printing papers conflicts with the continuous demands to decrease costs. The solution of this problem is especially urgent in the so-called LWC (lightweight coated) papers which are used for mail order catalogues, illustrated magazines, etc. The possibilities to achieve a decrease in cost by using less expensive raw materials without, at the same time, decreasing the quality have for the most part been exhausted. Today, LWC papers are produced with an extremely low weight per unit area to minimize costs.
The raw papers for coating have a weight between 36 and 40 g/m.sup.2, and the amount of coating used is between 8 to 12 g/m.sup.2 per side. These weights represent a technological limit below which one cannot go even with the greatest efforts. Moreover, work in these boundary areas leads to enormous difficulties during the coating process.
Thus, low weight per unit area often leads to "penetration" of the coating material, i.e., the coating material penetrates the substrate and builds up on the coating cylinder. This results in defects in the coating and, if there are heavier deposits, the paper tears. Then the coating device has to be stopped and cleaned. The downtime periods lead to increased production costs.
The penetration of the coating material into the paper has still another disadvantage. The coating material which penetrates into the paper does not contribute to the improvement in quality of the paper surface. Therefore, it is generally desirable that only that amount of coating material penetrate into the substrate which is needed to effect the bonding of the surface coating. However, the larger portion should remain on the surface to improve the printing capability. For this reason, coating materials which penetrate too deeply into those types of paper and cardboard with higher weight per unit area are also undesirable even though such penetration may not interfere with processing.
In the past, efforts to prevent excessive penetration of the coating material and especially penetration of the coating material in LWC papers have been made. Generally, there have been attempts to influence the penetrating behavior of the coating materials by increasing the solids content or the viscosity. However, these have not led to satisfactory results. The increase in the solids content in a coating material is limited by the water requirements needed for dispersion of the pigments used. Usually, coating materials with a solids content of more than 60% cannot be manufactured or at least cannot be used under conditions of actual practice. However, penetration of coating materials which have an even higher solids content can only be prevented to a limited extent.
An additional difficulty in LWC papers is that weights of the applied coating in the area of interest, i.e., below 12 g/m.sup.2, are difficult to achieve with coating materials having a solids content of more than 60%. Coating materials with only an increased viscosity also show no decreased penetration behavior. The addition of electrolytes and cationic substances to the coating material has been suggested, but usually such addition unfavorably affects the rheology of the coating material so that this modification has not been accepted in practice.